poly_int: SLP max_units

This match makes tree-vect-slp.c track the maximum number of vector
units as a poly_uint64 rather than an unsigned int.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-slp.c (vect_record_max_nunits, vect_build_slp_tree_1)
	(vect_build_slp_tree_2, vect_build_slp_tree): Change max_nunits
	from an unsigned int * to a poly_uint64_pod *.
	(calculate_unrolling_factor): New function.
	(vect_analyze_slp_instance): Use it.  Track polynomial max_nunits.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256127